1. Field of the Invention
The field of art to which this invention pertains is catalyst regeneration. More specifically, the present invention relates to a regeneration process for regenerating spent fluidizable catalytic cracking catalyst and a regeneration apparatus for use in the practice of the process.
2. Description of the Prior Art
In most regeneration processes presently employed the oxidation of coke from spent catalyst is done in a single-vessel regeneration zone containing one or more dense beds located in the bottom of the zone with a large dilute-phase disengaging space positioned above and in connection with the dense bed. In this type of regeneration process the dense bed is maintained in the bottom portion of the regeneration zone by limiting the superficial velocity of the incoming fresh regeneration gas to the transport velocity, that is, the velocity above which large amounts of catalyst would be carried out of the dense bed to the disengaging space. Typical velocities are therefore less than about 3 feet per second with 1.5 to 2.5 being the usual range. Any catalyst entrained in the flue gas effluent passing from the dense bed is recovered and returned to the same dense bed by passing this effluent flue gas containing entrained catalyst through separation means such as cyclone separation devices located in the disengaging space and directing separated catalyst to the dense bed. Average residence time of the catalyst within the regeneration zone per pass through the zone is generally in the two to five minute range with 2 to 3 minutes being the more common, while the residence time of gas is generally within the range of 10 to 20 seconds. All of the regenerated catalyst is returned directly from the regeneration zone to a hydrocarbon-reaction zone without additional passes through any part of the regeneration zone.
Most regeneration processes are also operated to essentially preclude significant CO combustion by controlling the oxygen-containing gas stream introduced into the process to maintain a rather small predetermined temperature differential between a flue-gas-outlet or a disengaging-space temperature and a dense-bed temperature within the regeneration zone. Excess oxygen within the regeneration zone is thus minimized thereby severely limiting CO combustion to only that amount characterized by the small temperature differential. Since the conversion of CO to CO.sub.2 is quite exothermic, this restricting of CO afterburning in most typical regeneration zones is done for the very practical reason of avoiding the damaging effects of excessively high temperatures in the upper disengaging space region of the regeneration apparatus where there is little catalyst present to act as a heat sink. This practice of admitting oxygen-containing gas into the process based upon a temperature differential, as exemplified by Pohlenz U.S. Pat. Nos. 3,161,583 and 3,206,391, produces a small amount of oxygen in the flue gas, generally in the range of about 0.1 to 1% oxygen, results in the flue gas containing from about 7 to about 14 vol. % CO and limits the temperatures achieved in the regeneration apparatus to a maximum of about 1275.degree. F. Typically, the flue gas from such processes is directed to the atmosphere where permitted by local air quality standards or to a CO boiler where it is used as fuel to generate steam.
The combination of limiting the superficial velocities within the regeneration apparatus and controlling the amount of fresh regeneration gas to eliminate significant CO combustion, which combination is employed in most prior art FCC regeneration processes, essentially fixes the degree of catalyst regeneration, that is, the amount of residual coke on regenerated catalyst. Although it is widely known that the residual coke content on regenerated catalyst has a great influence on the conversion and product distribution obtained from the hydrocarbon reaction zone, residual coke level on regenerated catalyst produced by most present regeneration processes conducted in conventional regeneration apparatus is not an independent variable but is fixed for each regeneration apparatus design at a level typically from about 0.1 to about 0.4 wt. % carbon.
Catalyst regeneration processes described in the recent prior art have recognized the advantages of essentially completely converting CO to CO.sub.2 and recovering at least a portion of the heat of combustion of CO both within the regeneration zone. Examples of such regeneration processes are Stine et al U.S. Pat. No. 3,844,973 and Horecky, Jr. et al U.S. Pat. No. 3,909,392. The advantages of such processes are now well known; such regeneration processes permit the reduction or elimination of feed preheat, the elimination of CO air pollution without the need for external CO boilers, and, when coupled with hydrocarbon-reaction zones of modern design, improved yields of more valuable products. In Stine et al U.S. Pat. No. 3,844,973 spent catalyst and fresh regeneration gas are passed into a first dense bed of a regeneration zone where coke is oxidized to produce regenerated catalyst and partially-spent regeneration gas which are passed in admixture through a transport riser wherein additional CO is oxidized to produce spent regeneration gas and heat of combustion is transferred to the regenerated catalyst. Hot regenerated catalyst and spent regeneration gas are separated and separated hot regenerated catalyst is passed to a second dense bed of catalyst from which it is returned to the hydrocarbon reaction zone. In the catalyst regeneration apparatus of Conner et al U.S. Pat. No. 3,893,812; Strother U.S. Pat. No. 3,898,050 and Pulak U.S. Pat. No. 3,953,175 a portion of the hot regenerated catalyst is returned from a second dense bed to a first dense bed via internal or external regenerated-catalyst recycle means to increase the temperature and hence the rate of coke oxidation in the first dense bed. The prior art has not, however, recognized the importance of properly mixing fresh regeneration gas, spent catalyst and the recycle hot regenerated catalyst so that the temperature in the first dense bed or coke oxidation zone is increased uniformly, fresh regeneration gas is used efficiently, and coke and CO oxidation proceed uniformly within the regeneration zone. Uniform coke oxidation is important in any regeneration zone to recover maximum activity from spent catalyst. Efficient use of fresh regeneration gas and uniform oxidization of coke and CO are particularly important in CO-burning regeneration processes where the refiner is usually concerned that the flue gas from the regeneration zone meet a specified air quality standard for CO. Poor mixing of spent catalyst and fresh regeneration gas results in uneven coke and CO oxidation which may require that the rate of fresh regeneration gas be increased beyond that required when there is good mixing in order to achieve a CO emission limitation. Besides reducing the coke-burning capacity of the regeneration zone an increased fresh regeneration gas rate may also increase the loading of the cyclone separation-devices in the regeneration zone thereby resulting in higher particulate emissions.